Preheat your oven to 375°F (190°C).
In a skillet over medium heat, add olive oil and sauté the minced garlic until fragrant, about 1 minute.
Add the chopped spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat.
In a mixing bowl, combine the cooked spinach, cream cheese, Parmesan cheese, mozzarella cheese, onion powder, salt, and pepper. Mix until well combined.
Using a sharp knife, carefully create a pocket in each chicken breast by cutting horizontally, ensuring not to slice all the way through.
Stuff each chicken breast with the spinach and cheese mixture. You can secure the opening with toothpicks or tie with kitchen twine.
Place the stuffed chicken breasts in a baking dish.
Bake in the preheated oven for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (75°C).
Remove from oven and let rest for 5 minutes before serving.